New kitty throwing up?

i just took this kitten in of the streets. i have not taken her to the vet yet. i started feeding her top of the line cat food and she starts throwing it up.thenshe just drops on her cot and is at a loss of energy. is this just her stomach getting used to the new food. if so how long should i allow this to go on before i get really alarmed. she is also having very frquent vowel movements.